The present invention is generally drawn to receivers for use with transmitters having High Power Amplifiers (HPAs), for example in satellite communications. Satellite communication systems must transmit signals vast distances from earth to satellites in orbit and vice-versa. A communication system may include a transmitter having an HPA or a transponder that includes a transmitter having an HPA. If a transmitter (or transmitter section of a transponder) is located in a space-based satellite, there is limited access thereto. Accordingly, compensating for the distortion associated with an HPA within a space-based satellite transmitter is much more complicated than compensating for the distortion associated with an HPA within a ground-based transmitter. Additionally, satellites in general have strict power consumption limits that require the communication systems to operate at very high efficiencies of both power use and usage of available communication bandwidth.
Many satellites use HPAs for communication purposes. Typically, HPAs operate most efficiently at (or near) saturation. Unfortunately, operation of HPAs at (or near) saturation can lead to inter symbol interference (ISI) and spectral spreading in output carriers.
The output of a transmitter can be seen as a sequence of symbols called a phrase. Each symbol represents a sequence of bits, in the case of 8PSK, each symbol represents 3 bits. A transmitter will output the phrase one symbol at a time during transmission. As a transmitter shifts from one symbol to the next in the phrase, previous and future output symbols may cause interference in the output of the current symbol. This interference in the current symbol caused by past and future symbols is ISI.
Due to physical limitations, there is a maximum number of HPA units that can fit in a transponder. Sharing multiple carriers by a single transponder HPA allows for transmitting more data and servicing more users without exceeding this physical limitation. Another benefit of this multicarrier operation is that it allows for reducing the transmission symbol rate per carrier without sacrificing system throughput. This greatly eases the burden on hardware implementation.
However, when multiple carriers are amplified by way of a single HPA, the HPA causes spectral spreading, wherein the information transmitted on one carrier can “bleed over” into information transmitted on an adjacent carrier, which is referred to as adjacent carrier interference (ACI). Ideally, the frequencies of adjacent carriers should be as close as possible. However, as the frequencies of adjacent carriers are brought closer together, many components of the ACI increase.
To further complicate matters, amplifying multiple carriers by way of a single HPA introduces nonlinear distortions, which include nonlinear components of ISI and nonlinear components of ACI, that further decrease information quality within each carrier. These nonlinear components of ISI and one component of nonlinear ACI are independent of the carrier spacing. In other words, the nonlinear components of ISI and one component of nonlinear ACI will not decrease even if the carriers are spaced far apart on a spectrum.
Conventional satellite communication systems with HPAs have been able to address ISI over a single carrier. Conventional satellite communication systems with HPAs that have addressed ISI have not been able to additionally correct for ACL Accordingly, conventional satellite communication systems with HPAs that have addressed ISI are not able to transmit over a plurality of carriers.
In essence, conventional satellite communication systems with HPAs are able to: drive the HPA in or near saturation while efficiently communicating over a single carrier; or inefficiently communicate over a plurality of carriers without driving the HPA in or near saturation.
What is needed is a method of compensation that addresses both linear and nonlinear ISI and linear and nonlinear ACI due to nonlinearity of HPA and tight crowding of carriers in a transmitter HPA or transmitter section of a transponder HPA, particularly when multiple carriers share a single HPA.